This invention relates to a new process for the production of urethanes by reacting aromatic amino compounds with alcohols and carbon monoxide in the presence of catalyst systems containing sulfur and/or selenium and/or compounds of these elements. Urethanes have generally been formed by reacting an aromatic isocyanate with an alcohol, the isocyanate in turn having been obtained by reacting phosgene with the corresponding primary amine. The amine was generally produced by reduction of the corresponding nitro compound. Unfortunately, this conventional process is attended by various disadvantages, not the least of which are the toxicity and the corrosive nature of phosgene and the formation of hydrogen chloride as a secondary product.
Accordingly, there have been numerous attempts to bypass the highly toxic phosgene and to produce the urethanes directly from the corresponding nitrogen compounds and the corresponding alcohols in the presence of carbon monoxide. For example, U.S. Pat. Nos. 3,338,956; 3,448,140; 3,454,620; 3,467,694; 3,531,512; 3,993,685; 4,052,420 and 4,052,437 and British Pat. Nos. 1,087,896; 1,080,094; 1,246,217; 1,469,222 or 1,472,243 relate to the synthesis of urethanes using catalyst systems containing noble metals of the platinum group or their compounds. U.S. Pat. Nos. 3,895,054 and 3,956,360 and British Pat. Nos. 1,485,108 and 1,486,399 relate to the synthesis of urethanes using catalyst systems containing selenium or compounds of selenium.
The production of urethanes from amino compounds, the corresponding alcohols and carbon monoxide using selenium as a catalyst is also known in the prior art (Chemistry Letters (1972), pages 373-374, published by the Chemical Society of Japan). In the process described in this literature reference, it is necessary to use stoichiometric quantities of selenium which results in considerable catalyst losses. Despite the use of relatively large quantities of selenium, the yields of urethane are very low when aromatic amines are used as the starting material. Furthermore, selenium and compounds of selenium are toxicologically unacceptable substances. In addition, selenium and its compounds leave the final urethane with an unpleasant odor.
The object of the present invention is to provide an improved process for the production of urethanes from aromatic amino compounds, alcohols and carbon monoxide in which either no selenium and/or selenium compounds are used or in which the quantity of selenium and/or selenium compounds can be considerably reduced.
This object is surprisingly achieved by the process according to the present invention.